Speed-changing mechanisms of many different types have long been well known and have been the subject of continuing development efforts. Those with which the present invention is concerned are of a wholly mechanical positive-drive class, in which special forms of "gearing" are uniquely exploited to promote very efficient, smooth and high-torque transmissions of power, while at the same time involing only relatively small bulk, minimum wear, and but few parts which desirably move at relatively slow speeds. The gearing preferred for such purposes comprises paired internally-toothed "sun" and externally-toothed "planet" gears in which substantially-continuous rolling contacts occur between many teeth which are simultaneously in torque-transmitting engagements. So-called "geroter" gears have such characteristics, and are described in U.S. Pat. No. 1,682,563 -- Hill, for example, where the interior gear has one less tooth than the exterior gear surrounding it. One such pair of gears has been described in the context of speed reduction and reversal, in U.S. Pat. No. 3,304,808 -- Grant. In another speed reducer, U.S. Pat. No. 2,874,594 -- Sundt, two sets or pairs of undulating-toothed gears are employed, with balls or rollers interposed between them, and with the "gears" being of different diameters. The latter fact, involving gear diameters which are different for the plural sets, represents a difficulty in relation to purposes of the present invention, and is found to be a characteristic of conventional forms of spur gearing, where different speed ratios are to be realized within the framework of like torque-transmitting capabilities. Spur gearing arrangements otherwise having some features of background pertinence to the disclosures here are believed to appear in U.S. Pat. No. 2,108,384 -- Moisy, and U.S. Pat. No. 3,056,315 -- Mros, and U.S Pat. No. 3,429,393 -- Lorence, and U.S. Pat. No. 2,667,089 -- Gregory, and U.S. Pat. No. 2,972,910 -- Menge. Further, my own U.S. Pat. No. 3,574,489 discloses certain orbital drives in which the gearing is of configurations akin to those preferred for the present purposes.
A form of hitherto-known gearing which lends itself especially well to uses in the mechanical drives of this invention includes an internal planet gear having "cycloidal" shape teeth or lobes of epitrochoid curvature cooperating with a surrounding external sun gear wherein the teeth are in the form of cylindrical pins or rollers and exceed by one the number of teeth of the planet gear. My improved and unusual mechanical drives feature an interchangeability of gearing sets, and related wide range of selectable speed changes, which are promoted through applications of at least two sun-and-planet gear sets wherein the interchangeable planet gears have the same diameter and the same mass eccentricities, and thus remain matched with counterweighting despite their different numbers of teeth, and wherein the cooperating sun gears are likewise all of the same diameter and have their teeth conveniently disposed at one predetermined radial distance from a central axis.